Some wireless networks, such as those complying with IEEE standard 802.11a/n, may use ‘wide’ channels with a bandwidth of 40 MHz, each wide channel composed of two legacy ‘narrow’ channels having a width of 20 MHz each. Some other wireless networks, such as those complying with IEEE standard 802.11ac, may use wide channels with a width of 80 MHz, each composed of four legacy 20 MHz channels. In both cases, before a device may transmit on a wide channel, it must determine that all the associated narrow channels are idle so that the transmission won't interfere with another device that might be using one or more of those narrow channels. To reduce the chance that multiple devices will try to seize the same idle channels at the same time and thereby try to transmit at the same time, each device waits for a randomly selected backoff period (as measured by a backoff counter) before transmitting on the wide channel, assuming the associated narrow channels are still idle after the backoff period. The maximum allowable backoff period may be defined by a ‘contention window’. A failure by a particular device to access the medium in this manner, a situation that might occur frequently in a heavily congested network, may result in increasing the size of the device's contention window and thereby increasing its average backoff time. Further, if a device senses that one of the channels becomes busy during the backoff countdown, the count may be restarted from the beginning. These two actions may help reduce network congestion, but they make it even less likely that the device will gain control of the channel quickly, since other devices with shorter backoff times will acquire control of the channel first.
When two separate networks with overlapping coverage areas are using some of the same narrow channels, the same protocols may be followed to reduce the chance that a transmission in one network will interfere with a reception in the other network. However, one network may be at a disadvantage in this situation. For example, if one network has a lot of data traffic, the contention windows of the devices in that network are likely to increase in size more than in the other network. Devices with larger average backoff times (due to the larger contention windows) are less likely to get the channel than devices with shorter average backoff times. Alternatively, devices that require four narrow channels are less likely to find all those channels idle than are devices that only require two narrow channels.